Lyme disease (LD) cases due to Borrelia burgdorferi (Bb) are increasing. Assays to detect active infection are urgently needed. The primary objective of this proposal is to develop an assay for active infection based upon Bb specific immune complexes (IC) for immediate public use. There are 2 Specific Aims to prove the principle and confirm that l). BbIC become positive before free Bb antibody (Ab) in early infection 2). BbIC indicate active infection Samples are already banked from LD patients with both the pathognomic erythema migrans (EM) rash AND microbiological confirmation by Bb cultures. The rationale is that in many infections IC Ab can be found bound to its antigen (Ag) target earlier than the free Ab, and in contrast to free Ab, specific IC Ab reflects active infection. We will study serial blinded serum specimens at key time points. ELISA, immunoblot and clinical histories will be available. BbIC will be isolated by a proven simple technique, polyethylene glycol. BbIC Ag and Ab will be examined by immunoblots and ELISA. Focus will be on specific and in vivo expressed Bb Ags. Results will be statistically analyzed. Phase 2 is designed for broadscale usage and refinement of the assay. PROPOSED COMMERCIAL APPLICATIONS: The need for accurate laboratory diagnosis of active infection in Lyme disease is imperative. There are over 5 million serologic Lyme disease tests performed per year. None indicate active infection. They only indicate past infection. A test such as the Borrelia burgdorferi immune complex assay has high potential as a marker of active infection. It can also detect infection early than free antibody assays. Therefore this assay can fulfill a great need in Lyme disease diagnosis and has high market potential.